Why does a bezier curve twist when rotated?
According to Wiki:


  A 3D Curve has Control Points that are not located on the Curve’s local XY plane. This gives the Curve a twist which can affect the Curve normals. You can alter how the twist of the Curve is calculated by choosing from Minimum, Tangent and Z-Up options from the drop-down menu.


There are three tilting calculation methods. Choose the one that results as expected.